1. Field of the Invention
The present invention relates to a slit exposure type copying machine, and more particularly to such a copying machine for copying with anamorphic magnification wherein the image of an original scanned in the form of a slit by a scanning system is projected on a photosensitive member through a projection lens and at least one triangular prism to form an image on the member.
2. Description of the Prior Art
Copying machines for giving varying copying magnifications are known wherein the projection lens is shifted to the position of magnification of .beta.X, with the scanning speed of the scanning system altered to V/.beta. (V: peripheral speed of the photosensitive member), to thereby obtain a copy at an altered magnification of .beta.X in each of vertical and horizontal directions.
However, copying machines of the type stated for copying anamorphic magnification have been proposed in recent years because they are useful for designing purposes in varying the vertical-to-horizontal ratio of characters and graphic figures, for forming a binding margin along only one vertical or horizontal side of copies or for eliminating defects in copy images when the forced separation method is used.
The term "anamorphic magnification" as herein used refers to a method of copying the image of an original at a different magnification in each of vertical and horizontal directions.
U.S. Pat. No. 3,445,161, for example, discloses a technique for giving a varied vertical-to-horizontal ratio by winding artist's copy and a film around drums and projecting the image of the copy on the film through a lens and a slit while rotating the copy and film drums at different speeds.
With reference to FIG. 33, it is now assumed that an original 2 placed on an original support 11 is to be scanned in the direction of arrow Y along the widthwise direction of a slit 3. For the following description, the widthwise direction of the slit is defined as the direction of arrow R parallel to the scanning direction Y, and the longitudinal direction of the slit as the direction of arrow J perpendicular to the scanning direction Y.
The technique of U.S. Pat. No. 3,445,161 will be applied to an electrophotographic copying machine in the following manner. The image of an original is scanned by a scanning system and then projected by a projection lens on a photosenstive drum rotating at a given speed. For example, the projection lens is brought to the position of magnification of .beta.1X, and the scanning system is set to a scanning speed of V/.beta.1.beta.2. The image then formed on the photosensitive drum has a magnification of .beta.1X in the slit longitudinal direction and a magnification of .beta.1.beta.2X in the slit widthwise direction. The image is developed and transferred to paper to afford a copy of anamorphic magnification.
With the above method, however, the peripheral speed of the photosensitive drum differs from the speed of the image moving on the drum as will be described below, so that the image projected on the drum becomes obscure, hence the drawback of reduced resolving power. In other words, it is impossible to obtain .beta.2X which differs greatly from 1X, such that the .beta.2X actually useful is limited approximately to 1.+-.0.1X.
The reduction of resolving power mentioned will be described with reference to FIG. 1 which shows the operation of a slit exposure type copying system wherein the original is adapted to travel. For giving anamorphic magnification, a projection lens 1 is placed at the position corresponding to a magnification of .beta.1X. An original 2 moves at a speed of V/.beta.1.beta.2 across a slit 3 having a width l. A photosensitive member 4 moves at a speed of V.
Now, the time t taken for a point A on the original 2 to move over the slit 3 is ##EQU1## The distance L the photosensitive member 4 moves during the time t is EQU L=Vt=.beta.1.beta.2l
During the time t, on the other hand, the point A of the original 2 moves to a point A', and the image formed by the projection lens 1 at the position of magnifiction .beta.1X moves from point B to point B' shown. The amount of movement, L', of the image is EQU L'=.beta.1l
Thus, while the point A of the original 2 moves to point A', the image moves from point B to point B', whereas a point C on the photosensitive member 4 moves to point C' (CC'=L). The difference between the image and the photosensitive member in the amount of movement results in a reduction in resolving power.
Accordingly, if the image is magnified at .beta.2X only in the slit widthwise direction, the amount of movement of the image is EQU .beta.2L'=.beta.1.beta.2l=L
Thus, no difference occurs between the two.
Published Examined Japanese Patent Application SHO No. 53-28087 discloses a cylindrical lens disposed in an optical path as means for eliminating the reduction of resolving power and having a refractive power only in the scanning direction. Nevertheless, the elongated cylindrical lens has the drawback of being difficult and costly to fabricate.